Snowbikes have become increasingly popular and ubiquitous for over-snow recreation over the past 10 years. Several manufacturers have designed, produced and marketed snowbike conversion systems (“kits”) to allow the seasonal conversion of motorcycles into snowbikes that ride and handle like motorcycles when ridden on snow. These motorcycle-like riding and handling characteristics have engendered a new powersport recreation genre with rapidly-growing popularity.
The majority of present snowbike kit designs employ track suspension designs previously used in snowmobiles. These snowbike kit designs typically utilize a suspension “tunnel” like a snowmobile that is an accessory frame that is rigidly mounted to the motorcycle chassis swingarm and shock absorber mounts to extend rearward from the motorcycle chassis. This frame mounts to the motorcycle chassis to form a rigid frame under which the track skid and suspension are deployed. Most of these snowmobile-like designs employ a parallelogram suspension configuration with one of the legs of the parallelogram having a variable length with minimum and maximum length limits. This allows the track skid to incline and decline relative the snowbike chassis to allow the track to maintain tractional compliance with the snow. An example of this style of snowbike suspension is disclosed in U.S. Pat. No. 8,910,738 {Mangum}.
These snowmobile-type suspension configurations typically limit the suspension compression to only 7 or 8 inches of travel at the forward end of their track skids. Additionally, the nature of these collapsing parallelogram suspensions yields a falling-rate suspension where the rate of shock absorber compression relative to the track skid compression decreases as the track suspension is compressed. This typically results in harsh bottoming of the track suspension when traversing very rough terrain or landing from jumping.
To remedy these shortcomings, some snowbike kit manufacturers have offered accessory shock absorbers that can replace the solid link connecting the tunnel frame to the motorcycle chassis shock mount. This additional shock absorber allows the track tunnel to rotate upward on the motorcycle chassis swingarm pivot to increase the track suspension range. While the geometry of the deployment of this additional shock absorber provides a slightly rising-rate compression of this additional accessory shock absorber, when combined with the under-tunnel track skid suspension, the net overall result is an overall generally-discontinuous falling-rate suspension.
Another style of snowbike conversion kit has been disclosed and described by the inventor of the present invention in U.S. Pat. Nos. 6,112,840, 6,321,864, 6,382,338 and 6,431,301 {Forbes}. This style utilizes a smaller track carriage having track drive sprockets at the rear of the track carriage frame and track idler wheels at the forward end, with the track circulating a constant path around the track carriage and being driven from the rear of the track carriage. A single articulating swingarm connects between the motorcycle swingarm pivot mount and the rear of the track carriage such that the track carriage such that the track carriage rises in scissors-fashion under the swingarm as the track suspension becomes compressed. The track carriage receives suspension cushioning force from the swingarm at the rear of the track carriage, and from a link connected to a bell crank lever (arm) to the front of the track carriage. A shock absorber is deployed between the shock absorber mount on the frame of the motorcycle and another arm of the bell crank such that the bell crank apportions the suspension cushioning force from the shock absorber to the swingarm and to the bell crank-connected link to the forward end of the track carriage. The drawbacks encountered in the product implementation of this design are first, the track is too short and too narrow, resulting in a track “footprint” too small for adequate floatation on soft snow, and second, the track suspension, while slightly progressive (or rising-rate), is of limited travel and lacks adequate progressivity of the suspension rate to provide the desired plushness of ride, and, like much of the other prior art, suffers from harsh bottoming of the track suspension.
Snowbike conversion kits are generally adapted to off-road racing corn petition-type motorcycles that are relatively light weight, have high horsepower-to-weight ratios, and have long travel front and rear wheel suspensions. As a result of many decades of design evolution, the rear wheel suspensions of these modern off-road motorcycles have continuously rising-rate (“progressive”) compression of their shock absorbers to provide nearly optimal suspension performance when riding over rough terrain and in landing from jumping. Off-road motorcycle owners who seasonally convert their motorcycles into snowbikes desire and seek the same suspension characteristics in their snowbikes that they enjoy in wheeled dirt bike configuration. Thus, the ideal snowbike track suspension will closely emulate the rising-rate suspension characteristics and performance of the rear wheel suspension of the host motorcycle.